We propose to conduct estimate how this association is modified by race/ethnicity, socioeconomic position, severity, age, and gender. Our hypothesis is that subgroups and/or disadvantaged populations will be more vulnerable to the effects of ambient air pollution. The study setting is Kaiser Permanente (KP) in Northern and Southern California, a large managed care organization with a base population of over 5.2 million people. Cases will be defined as members of KP who were hospitalized with a diagnosis of asthma (ICD-9 493) between 1997 and 2000 (n=11,800). Race/ethnicity, age and gender will be obtained from Kaiser Permanente databases, while socioeconomic data will be obtained by geocoding and linking to US census information. Severity will be derived from utilization databases in KP. Ambient air pollution and meteorologic data will be ascertained from ambient monitoring stations. Exposure and meteorologic factors will be mapped to the geographic areas and exposure assigned to each individual based on his or her residential address. Exposure related to the event for each individual will be compared to exposures on reference days using conditional logistic regression to estimate the risk of an asthma hospitalization associated with exposure to ambient air pollution. After adjusted for potential confounders, such as meteorology, and interactions with race/ethnicity, age, gender, disease severity, and socioeconomic position will be examined. This will be the first such epidemiologic study to look at most of these factors on a population-wide basis. This study will address a previously understudied question and if the results demonstrate that minorities and/or people living in lower SES areas are at higher risk for acute health effects from ambient air pollution, there will be research and policy implications.